1. Field of the Invention
The present invention relates to a hybrid circuit, more particularly to a hybrid circuit which is connected through a pair of lines B and A, i.e., subscriber lines, to a telephone terminal equipment, converts a receiving side four-wire signal to the telephone terminal equipment into a two-wire signal on the lines B and A and transmits the same, and converts a two-wire signal transmitted from the telephone terminal equipment to a transmitting side four-wire signal and transmits the same.
2. Description of the Related Art
A conventional, general, typical hybrid circuit consists of a transformer, as mentioned later in detail. However, if a transformer is used, there are limits to how small the hybrid circuit can be made and further there is the disadvantage of susceptibility to temperature fluctuations etc. Further, there is the disadvantage of greater deterioration of the characteristics at low frequencies. The biggest disadvantage of all is that it is difficult to make the transformer with an LSI.
Therefore, in recent years, attempts have been made to realize a hybrid circuit suitable for an LSI by making the transformer functions in the hybrid circuit by electronic components. A typical example of this is U.S. Pat. No. 4,004,109 to Boxall. The hybrid circuit patented by Boxaii, as will be explained with reference to the figures later, succeeds in electronically realizing the transformer function by connecting several mirror circuits to the lines A and B.
However, if use is made of the hybrid circuit of Boxall, a phenomenon of non-balance to the ground occurs in the in-phase signal (in general, the noise signal from the outside) superposed on the lines B and A due to the fact that it is impossible, from the current IC manufacturing technology, to achieve ideal characteristics for all the mirror circuits, i.e., to achieve mirror circuits all having the exact same electrical characteristics. In-phase signals affected by this non-balance to ground appear as differential signals on the lines B and A. These in-phase signals changing to differential signals become noise with respect to the real differential signals (two-wire signals constituting speech and other communication signals) transmitted on the lines B and A. This noise causes deterioration of the quality of the signal (speech) and must be suppressed as much as possible.
Note that another electronic hybrid circuit of a similar construction as Boxall is disclosed in FIG. 4 of U.S. Pat. No. 4,827,505.